The health and integrity of photoreceptors critically depend on the composition and volume of their extracellular microenvironment. Regulation of the ionic composition and volume of this so-called subretinal space is accomplished by the transport of ions, water, and metabolites across the retinal pigment epithelium (RPE), a monolayer of cells juxtaposed between the photoreceptor outer segments and the choroidal blood supply. RPE transport is the result of the coordinated activity of a diverse group of ion pumps, co-transporters , exchangers, and channels residing in the apical and basolateral membranes. With changes in retinal activity, chemical signals released by retinal cell diffuse too the RPE where transport is adjusted to compensate for alteration in the photoreceptor microenvironment. Disruption of these transport processes or their regulation may cause adverse changes in the subretinal space, contributing to retinal disease. These transport pathways are also responsible for maintaining the intracellular composition in the RPE cell, which, if disturbed, could adversely affect other key RPE functions such as vitamin A transport and metabolism. Our overall goal is to understand the mechanisms by which potassium (K+) channels participate in the regulation of the volume and ionic composition of the fluid in both the subretinal space and the RPE cytoplasm. The specific aims are: (1) To determine the molecular basis for the inwardly rectifying K+ (Kir) conductance of the RPE; (2) To determine the mechanism underlying the regulation of the Kir channel by intracellular ATP; (3) To understand how the Kir channel is modulated by physiological changes in intracellular pH; and (4) To test the hypothesis that volume-induced activation of another K+ channel, an M-type K+ channel, is mediated by arachidonic acid metabolites. These aims will be pursued using a combination of molecular and electrophysiological techniques to investigate K+ channel structure, function, and regulation. The outcome of these studies will be a better understanding of how these critically important transport proteins operate in the RPE to maintain a healthy photoreceptor microenvironment.